Controlling a polarization mode and a spatial mode of optical signals in photonic integrated circuits (PICs) is important for optical communication networks. For example, a conventional single mode optical fiber does not preserve the polarization mode. When the optical signal is coupled from a single mode optical fiber to the PICs, the signal decomposes into arbitrary compositions of two orthogonal polarization components, namely, a first component in a transverse electric (TE) mode, and a second component in a transverse magnetic (TM) mode. In many modules used in the PICs, the components in the TE and TM modes have different characteristics. For example, the components having different TE and TM modes propagate at different velocities in a high index contrast waveguide, and energy coupling coefficients of a micro-ring resonator for the TE and TM modes are different.
These polarization-dependent effects reduce the performances of the PICs, especially for high-speed communication. Also, most optical communication networks use only one polarization mode. Furthermore, if the components in both polarization modes are used in polarization-division multiplexing (PDM) systems, then the spectral efficiency of such systems can be increased.
Typically, systems for controlling polarization of optical signals, e.g., polarization transparent systems and polarization multiplexing systems, use various polarization manipulators, such as polarization converters and/or polarization splitters/combiners. For example, polarization splitters can be utilized in polarization transparent systems to solve, e.g., polarization dependence and polarization mode dispersion problems in the current photonic integrated circuits (PICs). Also, the polarization splitters can be utilized in polarization-division multiplexing (PDM) systems to increase the spectral efficiency.
Polarization splitters/combiners in PICs typically have very large size (length>1 mm), and requires processes specifically designed for these devices that make polarization splitters/combiners very complicated and expensive to fabricate.
For example, a polarization splitter based on a deeply etched multi-mode interference (MMI) waveguide is described by Rahman et al., “Design of optical polarization splitters in a single-section deeply etched MMI waveguide,” Applied Physics B, vol. 73, p. 613-619, 2001. In this case, a long (>2 mm) MMI with deeply-etched sidewall shows a small birefringence (i.e., different effective refractive indices between TE and TM modes). Therefore, the image of the input beam appears onto different output waveguides, depending on the polarization of the input beam.
In another example, a polarization splitter, based on two MMIs (one for 1×2 splitter, and the other for 2×2 coupler) and two waveguides connecting them, is described by Doerr (US 2010/0046886 A1). By choosing the different width for these waveguides such that TE and TM modes have different effective refractive indices, the input signal is guided to different output waveguides, depending on the input polarizations. Even though the actual device length is not specified in this application, it is conceivable that the total length exceeds 1 mm.
Accordingly, there is a need to reduce the length a polarization splitter or combiner.